The Organic Light Emitting Display (OLED) possesses many outstanding properties of self-illumination, low driving voltage, high luminescence efficiency, short response time, high clarity and contrast, near 180° view angle, wide range of working temperature, applicability of flexible display and large scale full color display. The OLED is considered as the most potential display device.
The OLED can be categorized into two major types according to the driving methods, which are the Passive Matrix OLED (PMOLED) and the Active Matrix OLED (AMOLED), i.e. two types of the direct addressing and the Thin Film Transistor (TFT) matrix addressing. The AMOLED comprises pixels arranged in array and belongs to active display type, which has high lighting efficiency and is generally utilized for the large scale display devices of high resolution.
The AMOLED is a current driving element. When the electrical current flows through the organic light emitting diode, the organic light emitting diode emits light, and the brightness is determined according to the current flowing through the organic light emitting diode itself. Most of the present Integrated Circuits (IC) only transmit voltage signals. Therefore, the AMOLED pixel driving circuit needs to accomplish the task of converting the voltage signals into the current signals. The traditional AMOLED pixel driving circuit generally is 2T1C, which is a structure comprising two thin film transistors and one capacitor to convert the voltage into the current.
As shown in FIG. 1, which is a 2T1C pixel driving circuit employed for an AMOLED, comprising a first P type thin film transistor T10, a second P type thin film transistor T20 and a capacitor C. The first P type thin film transistor T10 is a switch thin film transistor, and the second P type thin film transistor T20 is a drive thin film transistor, and the capacitor C is a storage capacitor. Specifically, a gate of the first P type thin film transistor T10 receives a scan signal Scan, and a source receives a data signal Data, and a drain is electrically coupled to a gate of the second P type thin film transistor T20 and one end of the capacitor C; a source of the second P type thin film transistor T20 receives a power supply voltage VDD, and a drain is electrically coupled to an anode of the organic light emitting diode D; a cathode of the organic light emitting diode D is grounded; one end of the capacitor C is electrically coupled to the drain of the first P type thin film transistor T10, and the other end is electrically coupled to the source of the second P type thin film transistor T20. As the AMOLED displays, the scan signal Scan controls the first P type thin film transistor T10 to be on, and the data signal Data enters the gate of the second P type thin film transistor T20 and the capacitor C via the first P type thin film transistor T10. Then, the first P type thin film transistor T10 is off. With the storage function of the capacitor C, the gate voltage of the second P type thin film transistor T20 can remain to hold the data signal voltage to make the second P type thin film transistor T20 be in the conducted state to drive the current to enter the organic light emitting diode D via the second P type thin film transistor T20 and to drive the organic light emitting diode D to emit light.
The aforesaid 2T1C pixel driving circuit employed for AMOLED according to prior art is highly sensitive to the threshold voltage drift of the drive thin film transistor. Along with the threshold voltage drift of the drive thin film transistor, the change of the current flowing through the organic light emitting diode is very large. Thus, the current flowing through the organic light emitting diode is unstable and the brightness of the organic light emitting diode is very nonuniform, which extremely affect the display effect of the pictures. For solving the aforesaid issue, it is necessary to add a compensation circuit to each of the pixels. The compensation means that the compensation has to be implemented to the threshold voltage of the drive thin film transistor in each pixel to make the current flowing through the organic light emitting diode irrelevant with the threshold voltage.